When an electric light bulb is turned on, a sudden incoming surge of electrical current encounters the relatively low resistance (e.g. 3 ohms) typically exhibited by the cold filament. The resistance of the cold filament may be only 1/41 of that of its resistance (e.g. 125 ohms) at operating temperature, so that the initial current surge is forty-one times that of the operating current. Consequently, a weak or worn filament will often burn out at the moment the bulb is turned on. To remedy this problem and prolong light bulb life, various heat-sensitive variable-resistance light socket insert devices are presently known. Typically, such devices include a thermistor element having contact surfaces on either side thereof, which is interposed between the light socket and bulb contacts. When it is cold (e.g. when the electricity is turned off), the insert device exhibits a resistance which is typically higher than the normal operating resistance of the bulb filament. Accordingly, when the light bulb is turned on, the initial current surge through the filament is reduced by the insert device. As the insert device is heated by the current, its resistance drops so that the normal operating current is delivered to the filament. By then, the filament has attained its normal operating resistance.
Occasionally, however, the variable resistance insert device does not make good contact with the central contact of the light bulb and/or that of the socket. For example, the light bulb may exhibit a relatively flat base which is contoured such that the central contact does not protrude sufficiently therefrom. Accordingly, when the bulb is fully screwed into the socket, the bulb center contact does not engage the upper contact surface of the insert device. Further, where the light socket central contact has become "tired" and lost its resiliency, it may tend to flatten and thus lose touch with the bottom surface contact area of the insert device. Such faulty connection with the light socket contact may also be caused by rivets which protrude from the base of the light socket and thereby interfere with seating of the insert device close enough to the socket contact so that the socket contact is unable to engage the bottom surface contact area of the insert device.